What is it?
Rebar simply provides text values for your the health and mana of your character, your pet (if you have one), and any party members next to their respective status bars (in the default Blizzard UI) rather than on top of them. It also provides the same values for your target and focus frames, and for the game tooltip, although in this case the values are on top of the corresponding bar.
For each frame type (player, pet, target, focus, party, or tooltip) you can individually control whether text is shown, and whether it is shown as deficit (how much is missing) or percentage value. For the target, focus, and tooltip frames, Rebar also recognizes friendly units (those that you can assist or cast heals on) and non-friendly units, so as to allow you to chose different settings for these.
Why write this?
In the 2.3 client, Blizzard added the option to show values or percentages on the player, pet, party, and target bars. However for me, the text of these values just gets in the way of being able to easily see where my status bar is at, and doesn't provide individual control for health and mana. It also doesn't provide a "deficit" mode to show how much health or mana is missing (something that I find very useful for healing).
How to use Rebar
Rebar remembers all of the settings you make in the options window on a per-character basis (since what is useful for your healer might not be so helpful for your DPSer). To display the Rebar Options window, just type /rebar or /reb.
Rebar has a number of settings that are global configurations, and don't have to be configured per character - font sizes per frame (set via the /rebar size command) and whether text should be shown all centered or not.
Acknowledgements
The CT team for CT_UnitFrames which I relied on for so long, and which I missed enough when they weren't around to write Rebar.
Enjoy!
-- Cirk